FAQ Radar

Which Radars work with Opencpn? What is best?

The following radar scanners are supported by plugins for OCPN version 4.8.4:

Garmin HD
Garmin xHD
Navico BR24 (All Navico are also sold as: Lowrance, Simrad, B&G)
Navico 3G
Navico 4G

For direct download to OCPN 4.8.4 see: https://opencpn.org/OpenCPN/info/downloadplugins.html [1446] And for wikis: https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn:opencpn_user_manual:plugins:ais_radar [1447]

There's ongoing development for a new radar plugin combining more types into one plugin; called radar_pi. This plugin can so far be used for all listed above except Garmin HD. Navico Halo is also supported but not thoroughly tested. (As far as is known)

Found here: Radar_pi [1448]

and can be built for both O 4.8.4 and O 4.99.0(Alfa for O 5.0 beta)

What type and maker is the best is of course up to each and everyone. I'm using radar_pi and Navico 3G and find it very good. Not less with the (M)ARPA function. The difference between 3G and 4G was not enough for the prize difference for me and the 20 NM range is good enough. Many opinions and configurations.

Another user has Navico 3G plugs straight into network port and working flawlessly with OCPN. If you choose this option depending on the “Branding” Lowrance, B&G, Simrad some are shipped with interface boxes and or interface cables. You do not need any of these if your intention is to connect directly to a PC running OCPN. Knowing this may save you some money.

The 3G radar plugin doesn't seem to work.

Setup: Radar ⇐= ethernet =⇒ Netgear FS105 Switch ⇐= ethernet =⇒ windows 10 laptop
Message: “Cannot switch radar on as it is either not connected or is not off.”
Suggestion: ENABLE OpenGL is required.
[] Accelerated Graphics (OpenGL) [must be checked!]

How can I connect the Radar's Ethernet to my USB port?

See the plugin documentation and
See thread [1449]

How are AIS and Marpa Calculated?

The plugin calculates AIS and Marpa completely autonomously.

AIS uses SOG, COG & GPS.
Marpa uses GPS location + time for own boat position. Heading for own boat heading in order to rotate the radar image relative to the boat. From this it computes relative distance and heading to a target and from that the true direction and position of all targets. The results are true speed and course of the target.
All calculations are in a message including position, course, speed, a ID number and target status(Acquiring/Active/Lost) transferred to OCPN as a $RATTM message. In OCPN this is evaluated by the AIS_Decoder function to a ARPA symbol.

=== I would like to have a better understanding how Marpa works with with collision avoidance, when the BR24_pi plugin only uuses/displays SOG and COG, when course through water or relative vector, is needed to calculate CPA and TCPA based on movements relative together, not on ground based movements. —

ARPA target movements are calculated based on the differences in GPS target locations over the various sweeps (spokes) of the radar.

This results in true target positions and speed.

OpenCPN handles these as AIS targets and calculates CPA and TCPA.
From CPA and TCPA the skipper knows if there is a danger or not.
When action is needed to avoid the danger he has to rely on relative observations of the danger to decide on what to do. For this he can use the relative radar picture with relative trails. He does not need to know speed through water as he already knows that there is a dangerous situation.
Using the relative picture he has to decide on the action to take. Once he has taken an action, he can use the new CPA and TCPA to judge if the action was sufficiently effective.
But on top of this, a radar image with trails relative to the water movement would help of course in the decision making. We don't have these (yet).

I thought that the option "Show relative target trails" will show the movements relative to me? So i´ll get the Information about the relative movement on the chartoverlay too (as target trail).

Yes. Relative trails will show you the movement of targets relative to you.
Relative trails are better shown and interpreted in the PPI (radar only) window.
It seems not logical to show relative trails on the chart overlay, which maps relative trails of a target on a true position and shows where the target has never been. We are now considering changing “Show relative trails to “Show True Trails” for the Chart Overlay, so that the target has indeed been in every location of the trail.

Should the Heading source connected via the Interface box or just connect it directly with the Ethernet RJ45 connection on the Opencpn computer? Which is better?

Doesn't matter much. Both way are used. And the course update frequency is not that crucial for normal sailing speed. I use my compass unit to feed OCPN at 1 Hz. Others are using a faster transfer via Navico I-box.
Send heading to the radar via the interface box so that heading is available in each spoke

correctly synchronised with when the radar generated the image. In practice the difference isn't that big.

Tupia reports, My radar is plugged directly into the RJ45 port on the computer and it has not had any problems. Uses the heading from fluxgate compass over NMEA. When setting up you need to accurately determine the offset angle while the boat is completely stationary and you have multiple identifiable targets.

Can we test the marpa function here in the garden at home? (no marine traffic). Have just tested the normal radar functions.

You can set a MARPA on a stationary target for test. Set a guard zone where just one or two blobs are present and check “ARPA On” or right click near a blob and “Acquire ARPA target”.